Conventionally, when mounting a resin casing on a base member such as a metallic bracket using threaded bolts, a plurality of tabs each having a hole are typically provided along the outer periphery of the resin casing, so that threaded bolts may be passed through these holes of the tabs and threaded with the corresponding threaded holes provided in the bracket for the purpose of mounting the resin casing on the bracket.
However, when threaded bolts are used, a certain fastening force is required in order to ensure the necessary security of attachment of the resin casing to the case member, but applying excessive force to the threaded bolts may cause damage to the resin case. Namely, there may be a case that the resin case is cracked or deformed by the excessive force.
In order to overcome such problems of the prior art, it was proposed in Japanese Utility Model Laid Open publication No. 56-136427 to absorb excessive fastening force by placing a buffer ring through which the threaded bolt is passed between the tab and the bracket. The buffer ring will be deformed or axially compressed when stress is applied thereto, whereby it was possible to conveniently absorb any excessive fastening force applied by the fastening of the threaded bolt.
However, according to such a structure, it was necessary for the buffer ring to be mounted on the year surface of the part to be mounted in alignment with the hole for passing the threaded bolt. Further, when the buffer ring is not in advance mounted on the rear surface, it was necessary for the buffer ring to be fitted to the stem portion of the threaded bolt which is passed through the hole of the tab. However, such processes are not always possible due to possible poor accessibility and other reasons. Furthermore, even when such a buffer ring is used, if the fastening force is excessive and the buffer ring is deformed beyond its elastic limit, damage may still be caused to the part of the resin member surrounding the hole of the tab for passing a threaded bolt.